R F output power control

ABSTRACT

An R.F. power amplifier circuit comprising a power control loop is described. The power control loop includes an R.F. power amplifier having a power control input and a power supply input as well as at least one feedback path coupled between the power control input and the power supply input of the power amplifier. The feedback path includes at least one variable loop element that has a control terminal configured to reduce variations of control loop parameters.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the field of radio frequency (R.F.)output power control and more particularly to an R.F. power amplifiercircuit comprising a power control loop including an R.F. poweramplifier. The invention further relates to a method of controlling theoutput power of the R.F. power amplifier.

2. Description of the Prior Art

Modern R.F. applications like wireless communications devices require anefficient R.F. output power control for example to ensure a hightransmission quality and to keep output signal fluctuations withinlimits that are defined in various standards.

Typically, R.F. output power control involves a power control loopincluding the R.F. power amplifier, a detector device for detecting theoutput power of the R.F. power amplifier and an error amplifier. Such anR.F. power control loop is known for example from U.S. Pat. No.5,378,996.

The power amplifier known from U.S. Pat. No. 5,378,996 has a controlinput, a signal input, an R.F. power output and a detector output. Thedetector signal at the detector output is provided by a detector devicein the form of a detector diode coupled between the R.F. power outputand the detector output of the power amplifier. The detector output iscoupled to a negative input of the error amplifier and an output of theerror amplifier is connected to the control input of the poweramplifier. Thus the power amplifier, the detector and the erroramplifier form a power control loop with negative feedback. The erroramplifier also has a positive input to which a reference signal isapplied.

The R.F. output power control described in U.S. Pat. No. 5,378,996 isbased on the measurement of the R.F. output power. However, in principleR.F. output power control could also be based on a measurement of theR.F. power amplifier current, i.e. on the power or current consumptionof the R.F. power amplifier.

R.F. output power control based on a measurement of the R.F. poweramplifier current is described in Ashok Bindra, “Smart biasing keepsR.F. power amplifier on track”, electronic design, 21 Jan. 2002, pp. 38,40. In this article a monolithic controller that regulates and controlsthe output power of an R.F. power amplifier is described.

The controller is part of a closed loop solution that permitscalibration of the R.F. power amplifier's gate biasing voltage inreal-time modes. A schematic block diagram of an R.F. power amplifiercircuit comprising the known controller is depicted in FIG. 1 a. Asbecomes apparent from FIG. 1 a, the R.F. power amplifier circuit 10comprises a power control loop 12 and a signal supply branch 14. Thepower control loop 12 comprises an R.F. power amplifier 22, a currentsensing element in the form of a resistor 24, a detector in the form ofa comparator 16 and a filter 18.

The resistor 24 is used to sense the drain current of the R.F. poweramplifier 22. The drain current is converted into a voltage that is fedtogether with an external voltage reference from the signal supplybranch 14 to the comparator 16.

The output signal of the comparator 16 is filtered by the filter 18 andthe filtered signal is used to control the R.F. power amplifier's 22gate biasing voltage.

To cope with temperature drift and aging that affect efficiency andlinearization of the R.F. power amplifier 22, a control input 14 ′ isprovided outside the power control loop 12 between the signal supplybranch 14 and the comparator 16. By means of a control signal applied tothe control input 14 ′ the output power POUT of the power amplifier 22can be controlled.

Departing from an R.F. output power control scheme taking into accountthe R.F. power amplifier current, there is a need for a R.F. poweramplifier circuit which allows a robust implementation of a powercontrol scheme. There is also a need for a method of controlling theR.F. power amplifier of such an R.F. power amplifier circuit.

SUMMARY OF THE INVENTION

According to the present invention an R.F. power amplifier circuit isprovided which comprises a power control loop including an R.F. poweramplifier having a power control input and a power supply input, and atleast one variable loop element coupled between the power control inputand the power supply input of the power amplifier, the at least onevariable loop element having a control input configured to reducevariations of control loop parameters.

The control input of the variable loop element thus enables to actuallycontrol the control loop by taking into account a feedback signalcharacteristic of a current consumption of the power amplifier. Thiscontrol of the control loop preferably aims at indirectly controllingthe output power by attaining a stationary state, whereas prior artsolutions aim at changing such a stationary state. The variable loopelement may be an element that can be tuned continuously or stepwise.The characteristics of the variable loop element arranged in thefeedback path may be controlled such that variations of dynamic loopparameters like the loop damping factor or the natural loop frequencyare reduced and, ideally, completely compensated.

It is thus firstly proposed to base the output power control on afeedback signal characteristic of the power amplifier current andsecondly to reduce loop parameter variations, many of which are specificto such a feedback mechanism, by providing one or more variable loopelements in the feedback path. The variable loop elements may beactively or passively controlled for example such that the control loopparameters become linearized or stationary.

Reduced control loop parameter variations lead to an output powercontrol which is more robust. Furthermore, calibration time requirede.g. for power-time-template calibration can be reduced especially inthe case of power amplifier circuits that are to be operated in multiplefrequency bands.

The variable loop element may be controlled directly or after signalconversion by a signal readily available at the power amplifier circuitand preferably by a signal related to the output power control like anexternally provided reference power control signal fed to the powercontrol loop or a power control signal created within the power controlloop. Additionally or alternatively, the variable loop element may becontrolled by a dedicated control signal like an offset signal.Preferably, the variable loop element is configured such that it issimultaneously controlled by a readily available signal related to theoutput power control and a signal related to the output power controland a dedicated control signal.

The variable loop element has a control input to which a dedicatedcontrol signal and/or a readily available but, if required, additionallyprocessed control signal may be fed. This control input allows forexample to tune the variable loop element continuously or discretely. Inparticular, the control input allows to create a further (internal)feedback path (i.e. an internal control loop for the variable loopelement) by coupling the control input for example to a particular nodeof the (external) feedback path between the power control input and thepower supply input of the power amplifier. Alternatively, the internalfeedback path may be created by coupling the control input of thevariable loop element to a node outside the external feedback path. Forexample the control input may be coupled to a signal supply branch ofthe power control loop. By means of the internal control loop a feedbacksignal tapped from the power control loop or the signal supply branchmay thus be fed directly or after signal conversion to the control inputof the variable loop element.

The control input of the variable loop element may be coupled to asignal converter which may be arranged in the internal feed back pathand which may comprise at least one of a filter circuit, a multiplier, alevel shifter, a buffer, a limiter, a look-up table and a voltage orcurrent source. Preferably, the signal converter converts a converterinput signal into a converter output signal that is coupled via thecontrol input to the variable loop element.

The converter input signal is preferably a readily available powercontrol signal or a signal derived therefrom. The signal converter mayhave his own control terminal to which for example the power controlsignal or the signal derived therefrom is fed. Alternatively, such asignal may be coupled directly to the control input of the variable loopelement. A digital control interface may be coupled either to thecontrol terminal of the signal converter or directly to the controlinput of the variable loop element. The digita control interface ispreferably arranged in the internal feed back path.

In a preferred embodiment the power amplifier is operable in multiplefrequency bands. In such a case the variable loop element may becontrolled in each frequency band differently. Such an individualcontrol is preferably performed such that identical loop parameters forall frequency bands are achieved. To that end, frequency band specificcontrol signals may be fed to the variable control element. Identicalloop parameters for all frequency bands allow to expedite calibrationsince calibration values found for one frequency band can be used (inconjunction with the appropriate control signal) for the remainingfrequency bands as well.

Variations of control loop parameters are caused by a plurality ofmechanisms. In the case the output power control is based on a feedbacksignal characteristic of the power amplifier current, variations of thepower amplifier constant are a major contribution to loop parametervariations. The power amplifier constant describes the relationshipbetween current consumption and control voltage of the power amplifier.

It is advantageous if the characteristic of the variable loop element isselected to vary and/or is varied in such a manner that the adverseeffects of variations of the power amplifier constant are reduced. Ofcourse, the characteristic of the variable loop element may also vary orbe varied such that additional effects or other effects that cause loopparameter variations are reduced.

The variable loop element may be a dedicated component arranged in thefeedback path solely for the purpose of reducing loop parametervariations. Additionally or alternatively, a component already presentin the feed path, for example a filter, a sensing element or a detector,may be configured such that the component allows in addition to itsprimary task a deliberate reduction of loop parameter variations.

Preferably, the variable loop element is constituted by a variablefilter like a loop filter or a low pass filter of the power controlloop. The variable filter may comprise at least one of a variableresistor and a variable capacitance. Furthermore, in the case of anactive filter a dedicated control routine may be implemented whichallows to reduce loop parameter variations.

The variable loop element may be constituted by or may comprise avaricap diode. Such a varicap diode provides a variable capacitancewhich is controlled by the voltage across the anode terminal and thecathode terminal. Thus the varicap diode may be controlled by the loopcontrol signal and more particularly by the loop control voltage. Anadditional control signal may be applied to either one or both of thetwo terminals of the varicap diode to introduce a further controlparameter.

If used in a filter arrangement, the varicap diode renders the filtervariable. However, the varicap diode may also be used in conjunctionwith other variable loop elements like a loop detector. Instead of or inaddition to a varicap diode, the loop detector may have a variable gainwhich is controlled such that loop parameter variations are reduced.

The invention described above may be implemented as a hardware solutionor as a software solution. In the case of a software solution theinvention may be realized in the form of a computer program productcomprising program code portion for performing the steps of theinvention. This computer program product may be stored on a computerreadable recording medium.

According to a preferred embodiment of the invention, the R.F. poweramplifier circuit of the invention is arranged in a network componentlike a mobile terminal for wireless communications (for example amulti-band mobile telephone) or a driver stage of a base station of amobile communications network.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and advantages of the invention will become apparentupon reading the following detailed description of preferred embodimentsof the invention and upon reference to the drawings, in which:

FIG. 1 a is a block diagram of a prior art R.F. power amplifier circuit;

FIG. 1 b is a block diagram of an R.F. power amplifier circuit accordingto the invention;

FIG. 2 is a diagram depicting the relationship between the poweramplifier constant and the power amplifier control voltage;

FIG. 3 shows a first implementation of a static loop filter;

FIG. 4 shows a second implementation of a static loop filter;

FIGS. 5 a and 5 b schematically show a first embodiment in accordancewith the invention of a variable loop element in the form of a variableloop filter;

FIG. 6 schematically shows a second embodiment in accordance with theinvention of a variable loop element in the form of a variable loopfilter;

FIG. 7 shows the relationships between the power amplifier controlvoltage and the variable capacitances of the two embodiments depicted inFIGS. 5 and 6;

FIG. 8 shows the variation of the damping factor in dependence of thepower amplifier control voltage for a prior art loop filter capacitorand a variable loop filter capacitor according to the invention;

FIG. 9 schematically shows a third embodiment in accordance with theinvention of a variable loop element in the form of a variable loopfilter;

FIG. 10 schematically shows a fourth embodiment in accordance with theinvention of a variable loop element in the form of a variable loopfilter;

FIG. 11 schematically shows a fifth embodiment in accordance with theinvention of a variable loop element in the form of a variable loopfilter;

FIG. 12 shows a block diagram of an R.F. power amplifier circuitcomprising the variable loop element depicted in FIG. 11 or 12;

FIG. 13 shows a practical arrangement of a variable loop filter in avariable loop filter;

FIG. 14 shows the relationship between the power amplifier output powerand the power amplifier control voltage for the arrangement depicted inFIG. 13; and

FIGS. 15 a and 15 b show the power amplifier output power as a functionof time step response of a prior art power amplifier circuit and of apower amplifier circuit according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following the invention is exemplarily set forth with respect toan R.F. power amplifier circuit comprising a variable loop element inthe form of a variable loop filter. It should be noted however that inprinciple any other element of the power control loop could be modifiedsuch that it functions as a variable loop element in addition to orinstead of the variable loop filter. Moreover, although the invention isexemplarily explained in conjunction with a variable loop capacitor,other variable components like variable resistors and other variableparameters like a variable detector gain may be used also to implement avariable loop element.

Furthermore, the following discussion of the preferred embodiments doesnot take temperature drift of the variable capacitors or requirementsfor properly biasing the variable capacitors into consideration. Inpractical realizations, appropriate means to compensate for thetemperature drift and means to properly bias the variable capacitorswill have to be provided.

In FIG. 1 b block diagram of an exemplary R.F. power amplifier circuit10 according to the invention is shown. The power amplifier circuit 10includes a power control loop 12 and a reference voltage supply branch14 coupled to the power control loop 12.

The power control loop 12 comprises a detector circuit 16, a loop filter18, an optional low pass filter 20, a power amplifier 22 and a currentsensing element 24. In the exemplary embodiment described with referenceto FIG. 1 b, the detector circuit 16 comprises an error amplifier havinga negative input 26 and a positive input 28. The positive input 28 iscoupled to the reference voltage supply branch 14 which includes a firstpulse shaping filter 32 in communication with the positive input 28 ofthe detector circuit 16, an exponential amplifier 30 having an outputcoupled to the first pulse shaping filter 32, and a second pulse shapingfilter 28 coupled to the input of the exponential amplifier 30.

An output of the detector circuit 16 is coupled to the loop filter 18which has a control input 46 configured to reduce loop parametervariations. The loop filer 18 is coupled via the low pass filter 20 to apower control input 34 of the power amplifier 22. The power amplifier 22further has a power supply input 36, an R.F. power output 38 and an R.F.signal input not shown in FIG. 1 b. The current sensing element 24 isconstituted by a resistor that is coupled between the power supply input36 of the power amplifier 22 and a current supply (V_(bat)).

The basic operation of the power amplifier circuit 10 depicted in FIG. 1b is as follows.

A discrete control voltage POWLEV* fed into the second pulse shapingfilter 28 is converted in the reference voltage supply branch 14 into acontinuous power amplifier reference voltage POWLEV that is applied tothe positive input 28 of the detector circuit 16. In the detectorcircuit 16 this power amplifier reference voltage POWLEV is comparedwith a feedback signal generated by the current sensing element 24. Thedifference signal is amplified by the detector circuit 16 and fed in theform of a power amplifier control voltage PAREG via the loop filter 18and the low pass filter 20 to the power control input 34 of the poweramplifier 22. The power amplifier 22 amplifies an R.F. input signal inaccordance with the power amplifier control voltage PAREG and outputsthe amplified signal via its power output 38.

As can be seen from FIG. 1 b, the power control loop 12 comprises afeedback path coupled between the power control input 34 and the powersupply input 36 of the power amplifier 22. This feedback path comprisesthe current sensing element 24, the detector circuit 16, the loop filter18 and the low pass filter 20.

By coupling the feedback path to the power supply input 36 of the poweramplifier 22, the output power control of the power amplifier 22 isbased on a feedback signal characteristic of the current consumption ofthe power amplifier 22. As a result of the fact that output powercontrol is based on a feedback signal characteristic of the currentconsumption of the power amplifier 22, the power amplifier constantK_(pa), which describes the relationship between current consumption andcontrol voltage PAREG of the power amplifier 22, influences the dynamicparameters of the power control loop 12. This becomes apparent from thetransfer function H(s) of the power control loop 12, which can bewritten $\begin{matrix}{{{H(s)} = \frac{{{Hf}(s)} \cdot {{Hlp}(s)} \cdot {Kdetector} \cdot {Kpa} \cdot {Ksense}}{1 + {{{Hf}(s)} \cdot {{Hlp}(s)} \cdot {Kdetector} \cdot {Kpa} \cdot {Ksense}}}},} & (1)\end{matrix}$where the transfer function Hf(s) of the loop filter 18 can exemplarilybe written as $\begin{matrix}{{{{Hf}(s)} = \frac{1}{s\quad{Cc}}},} & (2)\end{matrix}$the transfer function Hlp(s) of the low pass filter 20 can be written as$\begin{matrix}{{{{Hlp}(s)} = \frac{1}{1 + {s \cdot R_{lp} \cdot C_{lp}}}},} & (3)\end{matrix}$the transfer function K_(sense) of the current sensing element 24 can bewritten asK_(sense)=R_(senset)   (4)and the transfer function K_(detector) of the detector circuit 16 can bewritten asK_(detector)=G_(m)   (5)

In the following, two specific control loop parameters, namely the loopdamping factor d and the natural loop frequency w_(n), of the powercontrol loop 12 will exemplarily be considered in more detail. Thesecontrol loop parameters can be written as $\begin{matrix}{d = {\frac{1}{2} \cdot \sqrt{\frac{C_{c}}{C_{lp} \cdot R_{lp} \cdot K_{pa} \cdot R_{sense} \cdot G_{m}}}}} & (6) \\{and} & \quad \\{{wn} = \sqrt{\frac{R_{sense} \cdot K_{pa} \cdot G_{m}}{C_{lp} \cdot R_{lp} \cdot C_{c}}}} & (7)\end{matrix}$

From equations (6) and (7) it is obvious that dynamic properties likethe loop damping factor d and the loop bandwidth of the power controlloop 12 strongly depend on variations of the power amplifier constantK_(pa). The power amplifier constant K_(pa), however, strongly varieswith the power amplifier control voltage PAREG. This becomes apparentfrom FIG. 2 that shows a diagram depicting the functional relationshipbetween the power amplifier constant K_(pa) and the power amplifiercontrol voltage PAREG as derived on the basis of a power amplifiermodel.

It has been experimentally found that the power amplifier constantK_(pa) of a typical power amplifier for the 900 MHz band of the GlobalSystem for Mobile communications (GSM 900) varies between 1.6 A/V and0.96 A/V, that the power amplifier constant K_(pa) of a typical tripleband GSM 900/GSM1800/GSM1900 power amplifier ranges between 3 A/V and0.2 A/V, and that a dual band power amplifier has a maximum poweramplifier constant K_(pa) which can be as high as 6 A/V.

As becomes apparent from the above, the relationship between the poweramplifier control voltage PAREG and the power amplifier constant K_(pa)is highly non-linear. Consequently, typical dynamic control loopparameters like the loop damping factor d and the natural loop frequencyw_(n) strongly vary with the power amplifier control voltage PAREG.

Such variations of the control loop parameters render power control loopdesign very difficult. Power control loop design has to ensure that onthe one hand the loop bandwidth is wider than the bandwidth of the pulseshaping filters 28, 32 to ensure that pulse shaping remains independentof power control loop parameters. On the other hand the loop bandwidthshall be as small as possible (with constant damping factor) in order toreduce noise. Furthermore, constant control loop parameters areadvantageous from a calibration point of view. Power control loop designaims at finding a compromise on all aspects discussed above. Thisrequires, however, that variations of the control loop parameters arereduced as strong as possible.

According to the exemplary embodiment of the invention discussed incontext with FIG. 1 b, a variable loop element with a control input isprovided which reduces the loop parameter variations that result fromvariations of the power amplifier constant K_(pa). Of course, variationsof additional parameters or of other parameters apart from the poweramplifier constant K_(pa) could also be compensated in accordance withthe invention to linearize the control loop parameters.

In principle, variations of the power amplifier constant Kpa inequations (6) and (7) could be compensated by rendering one or more ofthe other parameters of equations (6) and (7) variable and be varyingthese one or more other parameters appropriately. This means thatbasically a variable loop element in the form of a variable currentsensing element 24, a variable detector circuit 16, a variable loopfilter 18 and/or a variable low pass filter 20 could be provided.Additionally or alternatively, a dedicated variable loop element couldbe introduced into the power control loop 12.

As becomes apparent from equations (6) and (7), the capacitance C_(c) ofthe loop filter 18 and the gain G_(m) of the detector circuit 16 areparameters that can especially advantageously be used for compensatingvariations of control loop parameters that are induced by a varyingpower amplifier constant K_(pa). In the following, linearization of thepower control loop 12 is exemplarily illustrated in conjunction with avariable capacitance C_(c).

Departing from the relationship between the power amplifier constantK_(pa) and the control voltage PAREG depicted in FIG. 2 on the one handand typical values for the individual parameters of equations (6) and(7) on the other hand, the damping factor d and the natural loopfrequency w_(n) can vary as illustrated in the following table: Gm 0.008S 0.008 S Cc 100 pF 100 pF Rlp 56 Ω 56 Ω Clp 1 nF 1 nF Rsense 0.05 Ω0.05 Ω Kpa 2.84 A/V (PAREG = 1.7 V) 0.03 A/V (PAREG = 2.5 V) d 2.0 22.3wn 4.501 × 10⁶ rad/s 4.401 × 10⁶ rad

The above variation of the damping factor d and of the natural loopfrequency w_(n) is made less dependent on the variation of the poweramplifier constant K_(pa) by introducing a variable loop filter having avariable capacitance C_(c) that can be varied at the same rate as thevariation of K_(pa). However, prior to discussing realization ofvariable loop filters, possible implementations of (static) realizationsare considered with reference to FIGS. 3 and 4.

According to a first variant, the detector circuit is realized in theform of an amplifier as part of a mixed signal ASIC, whereas the passiveloop filter is created with a discrete capacitor C and a discreteresistor R′ which form a PI loop filter as depicted in FIG. 3. Asbecomes apparent from FIG. 3, the discrete capacitor C is connectedsingle sided to ground.

The transfer function Hf(s) of the loop filter depicted in FIG. 3 can bewritten as follows: $\begin{matrix}{{{Hf}(s)} = \frac{1 + {s\quad R^{\prime}C}}{s\quad C}} & (8)\end{matrix}$

Equation (8) basically corresponds to equation (2) and equations (6) and(7) could be rewritten accordingly.

According to a second variant, the detector circuit and the loop filtermight be realized as depicted in FIG. 4, i.e. with an amplifier stage 40and a driver stage 42. The amplifier stage 40 and the driver stage 42are both part of a mixed signal ASIC, whereas loop filter capacitor Cremains a discrete component. The transfer function F(s) of the combinedamplifier stage 40 and driver stage 42 depicted in FIG. 4 can be writtenas $\begin{matrix}{{{Hf}(s)} = {G_{m} \times \frac{1}{s\quad C}}} & (9)\end{matrix}$

The implementation depicted in FIG. 4 is advantageous because comparedto the implementation depicted in FIG. 3 there is no external resistorrequired for the loop filter.

In FIG. 5 a a first variant of a variable loop filter 18 according tothe invention is depicted. As becomes apparent from FIG. 5 a, thevariable loop filter 18 comprises a resistor R′ and a variable loopcapacitor C_(var) in the form of a varicap diode. The loop capacitorC_(var) depicted in FIG. 5 a is the equivalent of loop capacitor Cdepicted in FIG. 3, but the capacitance of loop capacitor C_(var) isvariable and basically controlled by the power amplifier control voltagePAREG, i.e. changes approximately at the same rate as the poweramplifier constant K_(pa) varies.

The variable loop filter 18 has a control input 46 that is coupledbetween a cathode of the variable loop capacitor C_(var) on the one handand a common node to which the resistor R′ is coupled on the other hand.An external resistor R″ is also coupled to this common node and providesa control signal from the detector 16 to the control input 46 of thevariable loop filter 18. Thus an internal control loop including thevariable loop capacitor C_(var) is formed. An internal control signal ofthe control loop 12 is tapped via the resistor R″ and fed to the controlinput 46 coupled to the variable loop capacitor C_(var).

In principle, the two parallel resistors R′, R″ depicted in FIG. 5 acould be combined to a single resistor R. A corresponding equivalentcircuit of the variable loop filter 18 of FIG. 5 a is depicted in FIG. 5b.

The loop filter 18 depicted in FIGS. 5 a and 5 b constitutes a passivePI loop filter with variable “I” part due to the variable capacitorC_(var). The transfer function Hf(s) of the loop filter 18 becomes afunction of the voltage U_(cvar) across the terminals of C_(var). Thisallows to reduce variations of control loop parameters that are causedby variations of the power amplifier constant K_(pa).

FIG. 6 shows the same PI loop filter configuration as depicted in FIGS.5 a and 5 b but with an (additional) control terminal 46 for the loopcapacitor C_(var) that is coupled between an anode of the loop capacitorC_(var) and an additional capacitor C_(o) which provides a dc block toground. A control signal in the form of a control voltage U_(offset) maybe applied to the control input 46 of the variable loop filter 18. Inthe presence of U_(offset) the value of C_(var) _(—) _(offset) isdetermined by the difference between the power amplifier control voltagePAREG and the control voltage U_(offset). The control voltage U_(offset)thus allows to tune the loop capacitor C_(var) in order to even betterreduce variations of control loop parameters.

The transfer function Hf(s) of the loop filter 18 depicted in FIG. 6,which is a function of the voltage difference U_(cvar) between the poweramplifier control voltage PAREG and the control voltage U_(offset), canbe written as $\begin{matrix}{{{Hf}\left( {s,{{Uc}\quad{var}}} \right)} = \frac{1 + {s\quad R\quad{Ci}}}{s\quad{Ci}}} \\{with} \\{{{Ci}\left( {{uc}\quad{var}} \right)} = \frac{{Co}*{Cvar\_ offset}\left( {{Uc}\quad{var}} \right)}{{Co} + {{Cvar\_ offset}\left( {{Uc}\quad{var}} \right)}}}\end{matrix}$

As a result of the control voltage U_(offest) applied to the controlinput 46 of the variable loop filter 18, the C_(var) _(—) _(offset)versus power amplifier control voltage PAREG curve is shifted along thex-axis. Such a tuning is extremely useful for matching thecharacteristics of C_(var) _(—) _(offset) to the characteristics of thepower amplifier constant K_(pa).

The characteristics of the C_(var) and C_(var) _(—) _(offset) aredepicted in FIG. 7. As becomes apparent from FIG. 7, C_(var) _(—)_(offset) is the shifted replica of C_(var).

In FIG. 8 a comparison of the damping factors d for a loop filter havinga fixed capacitance C and a variable loop filter 18 as depicted in FIG.6 having a tuned and variable loop filter capacity C_(var) _(—)_(offset) is shown. The use of the variable loop filter 18 allows toreduce the variation of the damping factor d by approximately a factorof 2. This also becomes apparent from the table below. Cc = Cvar_offsetCc = const Kpa 2.84 A/V (PAREG = 1.7 V) D 1.6 2.0 Wn 5.601 × 10⁶ rad/s4.501 × 10⁶ rad/s Kpa 0.03 A/V (PAREG = 2.5 V) D 10.5 22.3 Wn 0.852 ×10⁶ rad/s 0.401 × 10⁶ rad/s d_ratio 6.6 11.2 wn_ratio 0.152 0.089

Further approvements can be achieved by using a varicap diode with acapacitance characteristic that better matches the characteristic of thepower amplifier constant K_(pa) and by actively controlling the controlvoltage U_(offset).

Since the power amplifier constant K_(pa) also varies with frequency,the characteristics of the power amplifier constant K_(pa) will bedifferent for different frequency bands. The control voltage U_(offset)can thus be used to tune C_(var) _(—) _(offset) for each frequency bandindividually to achieve identical control loop parameters for allfrequency bands. In the case of identical control loop parameterspower-time-template calibration for multiple frequency band mobiletelephones is expedited because calibration values found for onefrequency band can readily be used (if the appropriate control voltageU_(offset) is applied) for the remaining frequency bands as well.Consequently, the calibration time might be reduced by more than 50% fora triple band mobile telephone.

An additional resistor R_(c) could be added, for example for filteringpurposes, to the variable loop filter 18 as shown in FIG. 9. Theresistor R_(c) is coupled to the control terminal 46 and a modifiedcontrol voltage U_(offset*) has to be applied to the resistor R_(c).

As depicted in FIG. 10, the control signal U_(offset*) can be providedby a signal converter 50 which helps to better adapt the characteristicof C_(var) _(—) _(offset) of the varicap diode to the characteristic ofthe power amplifier constant K_(pa). The signal converter 50 comprises acontrol terminal 52 for receiving a control signal U_(offset**) and anoutput terminal 54 coupled to the resistor R_(c). In principle, theresistor R_(c) depicted in FIG. 10 could be omitted and the outputterminal 54 of the signal converter 50 could be directly coupled to thecontrol input 46.

The signal converter 50 linearity or non-linearily transforms thecontrol signal U_(offset**) into the control signal U_(offset*) inaccordance with the relationship U_(offset*l =f)_(converter)(U_(offset**)). The signal converter 50 may comprise afilter circuit, a multiplier, a level shifter, a buffer, a limiter, alook-up table or a voltage/current source.

An alternative embodiment of the variable loop filter 18 with C_(var)_(—) _(offset) connected single sided to ground is depicted in FIG. 11.

In principle, the control input 46 of the variable loop filter 18 couldbe connected directly or indirectly to the power amplifier controlvoltage PAREG or another power control signal like the power amplifierreference voltages POWLEV or POWLEV*. In this regard FIG. 12 exemplarilyshows the block diagram of the power control loop 12 with the controlterminal 52 of the signal inverter 50 coupled to the PAREG signal tocreate an internal control loop 12′. It should be noted that the controlinput 46 of the variable loop filter 18, or of any other variable loopelement (for example the current sensing element 24 or the detector 16),could alternatively be coupled to a node arranged between the pulseshaping filter 28 and the detector 16 or the detector 16 and the loopfilter 18. Of course, U_(offset), U_(offset*) or U_(offset**) could alsobe supplied directly from a digital control interface like ananalog/digital converter.

FIG. 13 shows a circuit with a variable loop filter used for practicalmeasurements performed on a mobile telephone board. The circuit of FIG.13 is based on the circuit of FIG. 4 with the static loop filter.

Returning to FIG. 13, resistor R_(d) is used to provide a dc path forthe varicap diode C_(var). Capacity C_(dc) is used to provide dcdecoupling for the signal U_(offset**). The sigal U_(offset**) issupplied by an external, variable voltage source. The diagram of FIG. 14shows the power amplifier output power versus power control voltagePOWLEV as measured for the circuit of FIG. 13. From FIG. 14 it becomesapparent that the loop filter arrangement depicted in FIG. 13 helps toreduce the overshot in output power compared to the unmodified circuitdepicted in FIG. 4.

FIGS. 15 a and 15 b show the output power versus time step response ofthe unmodified and modified circuit depicted in FIGS. 4 and 13,respectively. It can be clearly seen that the maximum power overshot isreduced by about more than 75%.

1-18. (canceled)
 19. An R.F. power amplifier circuit comprising a powercontrol loop with: an R.F. power amplifier having a power control inputand a power supply input; and at least one variable loop element coupledbetween the power control input and the power supply input of the poweramplifier, the at least one variable loop element having a control inputconfigured to reduce variations of control loop parameters.
 20. Thepower amplifier circuit of claim 19, wherein the control input of thevariable loop element is coupled to the power control loop or to asignal supply branch of the power control loop to form an internalcontrol loop which includes the variable loop element.
 21. The poweramplifier circuit of claim 19, wherein the variable loop element is avariable filter or a variable current sensing element.
 22. The poweramplifier circuit of claim 21, wherein the variable filter comprises atleast one of a variable resistor and a variable capacitance.
 23. Thepower amplifier circuit of claim 19, wherein the variable loop elementcomprises a varicap diode.
 24. The power amplifier circuit of claim 19,wherein the variable loop element is a detector having variablecharacteristics.
 25. The power amplifier circuit of claim 19, furthercomprising at least one of a digital control interface and a signalconverter coupled to the control input of the variable loop element. 26.The power amplifier circuit of claim 25, wherein the signal converter isarranged within an internal control loop which also includes thevariable loop element and which is formed by coupling the control inputof the variable loop element to the power control loop or to a signalsupply branch of the power control loop.
 27. The power amplifier circuitof claim 25, wherein a control terminal of the signal converter iscoupled to the digital control interface.
 28. The power amplifiercircuit of claim 25, wherein at least one of the signal converter andthe digital control interface is arranged within an internal controlloop which also includes the variable loop element and which is formedby coupling the control input of the van I able loop element to thepower control loop or to a signal supply branch of the power controlloop.
 29. A power control loop for an R.F, power amplifier circuit, thepower control loop comprising: an R.F. power amplifier having a powercontrol input and a power supply input; and a variable capacitancecoupled between t power control input and the power supply input of thepower amplifier; the variable capacitance having a control input forinputting a control signal for linarizing control loop parameters.
 30. Apower control loop of an R.F. power amplifier circuit, the power controlloop comprising: an R power amplifier having a power control input and apower supply input; at least one variable element arranged between thepower control input and the power supply input of the power amplifier,the at eat one variable element having a control input; and an internalcontrol loop including the variable element, the internal control loopbeing formed by coupling the control input of the variable element toeither one of the power control loop or a signal supply branch for thepower control loop.
 31. A method of controlling the output power of anR.F. power amplifier using a power control loop for taking into accounta feedback signal characteristic of a current consumption of the poweramplifier, comprising: providing at least one loop element with variablecharacteristics, the at least one loop element having a control input;varying the characteristics of the loop element by applying a controlsignal to the control input of the variable loop element such thatvariations of control loop parameters are reduced.
 32. The method ofclaim 31, wherein the characteristics of the loop element are varied toreduce variations of control loop parameters that result from variationsof the power amplifier constant.
 33. The method of claim 31, wherein afeedback signal tapped from the power control loop or from a signalsupply branch of the power control loop is directly or after signalconversion fed to the control input of the loop element.
 34. The methodof claim 31, wherein the characteristics of the loop element arecontrolled by a power control signal for the power amplifier or a signalderived therefrom.
 35. The method of claim 31, wherein thecharacteristics of the loop element are controlled by a dedicatedcontrol signal.
 36. The method of claim 31, wherein the R.F. poweramplifier is operable in multiple frequency bands and wherein thecharacteristics of the loop element are individually con trolled in eachfrequency band.
 37. A computer program product for controlling theoutput power of an R.F. power amplifier using a power control loop fortaking into account a feed back signal characteristic of a currentconsumption of the power amplifier, wherein at least one loop elementwith variable characteristics and with a control input is provided, thecomputer program product comprising pro gram code portions for varyingthe characteristics of the loop element such that variations of controlloop parameters are reduced.
 38. The computer program product of claim37, stored on a computer readable recording medium.